Polyurethane elastomers are becoming increasingly important commercial polymer materials. Unlike many other thermoplastic and thermoset elastomers, the spectrum of raw materials used in polyurethane elastomer formulations allows the preparation of elastomers with physical properties geared to the application to an extent not possible with many other polymers. Thus, polyurethane elastomers have a myriad of diverse applications, including caulks and sealants, elastomeric fibers, viscoelastic and energy absorbing materials, and extrusion and injection moldable articles such as gears, automobile fascias, and ski boots, to name but a few.
The unique properties of polyurethane elastomers are attributable, at least in part, to the somewhat unique polyurethane polymer morphology which includes both hard and soft segments in the same polymer molecule. By adjusting the nature and relative amounts of hard and soft segments, as well as the degree of cross-linking, elastomers may be produced with hardnesses from the low Shore A range to the high Shore D range. The hard and soft segment content also influence a number of other properties such as low temperature flexibility, tensile strength, modulus, and high temperature properties such as heat sag and distortion.
Polyurethane elastomers may be prepared by a number of methods. However, the preferred method involves the preparation of a soft-segment, isocyanate-terminated polyurethane prepolymer by reacting an excess of a di- or polyisocyanate with an isocyanate-reactive component of somewhat high molecular weight, for example a molecular weight in the range of 1000 Da to 4000 Da. The higher molecular weight isocyanate-reactive components generally result in a softer elastomer with greater elongation but lower tensile strength and modulus. The resulting isocyanate-terminated prepolymer is then chain-extended with a lower molecular weight isocyanate-reactive component, generally a glycol or diamine, to form the elastomer product.
Among the most useful polyurethane elastomers are glycol extended polyurethanes containing soft-segments based on polytetramethylene ether glycol (PTMEG). PTMEG has a repeating unit containing four methylene units linked through oxygen atoms, the general formula of which may be represented as: EQU HO--[CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 --O--].sub.n --H
where n is such that the PTMEG has a molecular weight of from 250 to 1000, preferably from 1000 to 4000. However, PTMEG is more expensive than other soft-segment polyols such as polyoxypropylene glycols and polyester diols, and thus for PTMEG products to be competitive, they must provide improved properties with respect to the desired application. The polyurethane industry has continually sought methods to increase the performance and lower the cost of PTMEG-based polyurethane elastomers, as well as to develop competing elastomers with similar properties but at lower cost by substituting other isocyanate-reactive polymers for PTMEG in the soft segments of the polyurethane elastomer.
For example, in U.S. Pat. No. 5,340,902 and in the article by A. T. Chen et al., "Comparison of the Dynamic Properties of Polyurethane Elastomers Based on Low Unsaturation Polyoxypropylene Glycols and Poly(tetramethylene oxide) Glycols," POLYURETHANES WORLD CONGRESS 1993, Oct. 10-13, 1993 (pp. 388-399), it is disclosed that polyurethane elastomers prepared from polyoxypropylene diols having unsaturation in the range of 0.014-0.017 meq/g have enhanced properties as compared to PTMEG based elastomers in the same Shore hardness range.
As discussed in the latter reference, preparation of polyoxypropylene polyols by traditional base catalyzed polymerization of propylene oxide (methyloxirane) onto a suitably functional initiator molecule, for example propylene glycol or trimethylolpropane, results in a product whose unsaturation and monol content rapidly increases with increasing molecular weight. The mechanism of formation of unsaturation is still subject to question, and has been discussed, for example, in BLOCK AND GRAFT POLYMERIZATION, V.2, Ceresa, Ed; John Wiley & Sons, on pages 17-21. Whatever the mechanism, the result is the formation of allyloxy groups which are monofunctional and which result in the formation of polyoxypropylene monols with allylic terminal unsaturation. Since the polyoxypropylene monols are lower molecular weight, monofunctional species, their presence alters both the functionality of the finished polyol as well as the molecular weight distribution.
The unsaturation created during base catalyzed propylene oxide polymerization may be measured by titration with mercuric acetate using the protocol established by ASTM D-2849-69, "Testing Urethane Foam Polyol Raw Materials." With polyoxypropylene glycols, for example, the unsaturation increases from about 0.027 meq. unsaturation/g polyol at an equivalent weight of 600-700 to in excess of 0.09 meq/g at an equivalent weight of c.a. 2000. These levels of unsaturation correspond to c.a. 10 mol percent in low molecular weight diols, and 50 mol percent of monol in a 4000 molecular weight polyoxypropylene glycol! The average functionality, at the same time, decreases from about 1.9 to about 1.6 at these levels of unsaturation, as compared to the theoretical functionality of 2.0.
In U.S. Pat. No. 5,340,902 and the Chen et al. article previously cited, polyoxypropylene polyols were prepared using a zine hexacyanocobaltate.glyme catalyst instead of traditional KOH catalyst. By this method of preparation, the unsaturation of a c.a. 4000 molecular weight polyoxypropylene glycol is reduced to levels in the range of 0.015 meq/g, corresponding to approximately 4 mole percent monol and an average
functionality in excess of 1.9. The authors disclose that polyoxypropylene glycols having a molecular weight of 2,250 can be used to prepare chain extended elastomers in the Shore A 80 hardness range with properties similar to those prepared from 1000 molecular weight PTMEG. Unfortunately, a direct comparison cannot be made, as the polyoxypropylene-prepared elastomers do not have identical hardnesses as their PTMEG counterparts. Differences in hardness affect many properties, most notably tensile strength, elongation, and modulus. Furthermore, to the extent they are comparable, the polyoxypropylene glycol-derived elastomers demonstrated lower split-tear strength than PTMEG products.
The ability to prepare PTMEG competitive polyoxypropylene glycol-based polyurethane elastomers utilizing low unsaturation polyols prepared by double metal cyanide glyme catalysis is premised on the low monol content of these polyols. The '902 patentees, for example, indicate that the normally relatively high monol content of conventional polyols act as chain terminators in polyurethane formation, limiting the elastomer molecular weight and downgrading physical properties. Elimination or reduction of monol content raises polyol functionality and increases elastomer molecular weight, thus increasing physical properties.
That the monol content in polyurethane elastomer formulations act as chain stoppers is further supported, for example, by U.S. Pat. No. 3,483,167, wherein a lower alkanol is added to a linear urethane prepolymer and then amine extended. The lower alkanol addition is stated to lower viscosity of the prepolymer. Similar results are reported in DE 3,132,760 A1. In U.S. Pat. No. 3,350,361, lower alkanols or monohydroxyl-functional polyethers are added to solution-borne polyurethane prepolymers prepared from polyoxyalkylene triols useful for treating leather. The monol is said to serve as chain blockers, increasing the plasticity of the crosslinked product. In U.S. Pat. No. 3,384,623, lower alkanols and monohydroxyl-functional polyesters and polyethers are added in solution to an uncatalyzed linear polyurethane prepolymer which is diamine extended to form a spandex spinning dope. The monols are stated to reduce gelling in the polymer solution after diamine addition. Neither of the latter patents provide compositions useful for cast polyurethane elastomers.
In U.S. Pat. No. 3,875,086, up to 50 weight percent hydroxyl-functional polyoxyalkylene polyether monols are added to the B-side of a one-shot polyurethane elastomer formulation containing significant amounts of polyoxyalkylene triols to control hardness and improve reactant flowability in production of flexible polyurethane foams. The elastomers prepared by the disclosed process were far softer and exhibited lower tensile and tear strengths than comparative products not containing the monols.
It would be desirable to prepare PTMEG products having enhanced properties as compared to those commercially available. It would further be desirable to lower the cost of PTMEG-based elastomers.